The present invention relates to a semiconductor memory device, and more particularly to a dynamic random access memory cell having an improved fin-structured storage electrode and a method of fabricating the same.
Generally, the dynamic random access memory cell has a pair of a transfer field effect transistor and a memory cell capacitor. This simple structure of the dynamic random access memory cell is suitable for high integration of the dynamic random access memory device. There has been developed a memory cell capacitor having a three dimensional structure for further increase in the density of the integration of the memory cell array in the dynamic random access memory device.
Namely, in the light of the scale reduction of the memory cell and increase in the density of the integration of the memory cell array, it is required to reduce an occupied area of the memory cell capacitor and ensure a large capacitance necessary for allowing the dynamic random access memory device to show stable operations and has a reliance on operations thereof, for the purpose of which it is further necessary to increase in surface area of a memory cell capacitor storage electrode as much as possible.
The three dimensional structured memory cell capacitor is classified into two types. First one is the stacked memory cell capacitor and second one is the trench memory cell capacitor. The stacked memory cell capacitor has a high resistivity to noises due to alpha-ray incidence or circuits, for which reason the stacked memory cell capacitor is capable of performing stable operations even if the capacitance of the stacked memory cell capacitor is relatively small. Generally, it seems considered that the stacked memory cell capacitor is available to 0.15 micrometers scale rule for 1 Gbit dynamic random access memory device.
As one of the stacked memory cell capacitors, various fin-structured memory cell capacitors have been proposed and, for example, disclosed in INTERNATIONAL ELECTRON DEVICES MEETING, 1988 pp. 593-595, entitled "3-DIMENSIONAL STACKED CAPACITOR CELL FOR 16M AND 64M DRAMS", and also disclosed in the Japanese laid-open patent publication No. 5-291524. The fin-structure is effective to increase the surface area of the memory cell capacitor storage electrode. In order to obtain a further increase in the surface area of the fin-structured storage electrode of the stacked memory cell capacitor, it is effective to increase the number of fins of the storage electrode. Since each fin comprises a conductive layer, if the number of the fines of the storage electrode is increased, then the mechanical strength of the fin structure of the storage electrode of the memory cell capacitor is reduced whereby the conductive film forming each fin may be curved. As a result, the reliability of performances of the fin structured storage electrode of the stacked memory cell capacitor is reduced. In the Japanese laid-open patent publication No. 5-291524, it is disclosed to strengthen the fin structure of the storage electrode of the memory cell capacitor. The fin-structured storage electrode of the stacked memory cell capacitor may be fabricated in accordance with a method to be described below with reference to FIGS. 1A through 1E which are fragmentary cross sectional elevation views illustrative of a conventional fin-structured storage electrode of the stacked memory cell capacitor.
With reference to FIG. 1A, a first silicon oxide film 52 is formed as an inter-layer insulator over a silicon substrate 51. A first silicon nitride film 53 is formed over the first silicon oxide film 52. The first silicon nitride film 53 will serve as an etching stopper to etchant of hydrofluoric acid in later process to be described below. A second silicon oxide film 54 is formed over the first silicon nitride film 53. A second silicon nitride film 55 is then formed over the second silicon oxide film 54. A first polysilicon film 56 is then formed over the second silicon nitride film 55. A third silicon oxide film 57 is formed over the first polysilicon film 56. A third silicon nitride film 58 is further formed over the third silicon oxide film 57. A second polysilicon film 59 is then formed over the third silicon nitride film 58. A fourth silicon oxide film 60 is then formed over the second polysilicon film 59. A fourth silicon nitride film 61 is then formed over the fourth silicon oxide film 60.
With reference to FIG. 1B, a contact hole 62 is formed, which vertically extends from the fourth silicon nitride film 61 to the first silicon oxide film 52 so that a part of the silicon substrate 51 is shown through the contact hole 62.
With reference to FIG. 1C, a third polysilicon film 63 is entirely formed over the fourth silicon nitride film 61 and within the contact hole 62 so that the silicon substrate 51 is made into contact with the polysilicon film 63.
With reference to FIG. 1D, the laminations of the second silicon nitride film 55, the first polysilicon film 56, the third silicon oxide film 57, the third silicon nitride film 58, the second polysilicon film 59, the fourth silicon oxide film 60, the fourth silicon nitride film 61 and the third polysilicon film 63 are subjected to an anisotropic etching to pattern the same.
With reference to FIG. 1E, by use of a hydrofluoric acid solution is used to carry out a wet etching or an isotropic etching to etching the second silicon oxide film 54, the third silicon oxide film 57 and the fourth silicon oxide film 60, wherein the first silicon nitride film 53, the second silicon nitride film 55, the third silicon nitride film 58 and the fourth silicon nitride film 61 serve as etching stoppers thereby to form a fin-structured storage capacitor electrode 64 over the silicon substrate 51. The first polysilicon film 56 serves as a first conductive layer of the fin-structured storage capacitor electrode 64. The first conductive layer is supported by the second silicon nitride film 55. The second polysilicon film 59 serves as a second conductive layer of the fin-structured storage capacitor electrode 64. The second conductive layer is supported by the third silicon nitride film 58. The third polysilicon film 63 serves as a third conductive layer of the fin-structured storage capacitor electrode 64. The third conductive layer is supported by the fourth silicon nitride film 61. In this case, the fin-structured storage electiode has three fins. Notwithstanding, it is possible to increase the number of fins of the fin-structured storage electrode in order to increase the surface area of the fin-structured storage electrode.
As described above, the second silicon nitride film 55, the third silicon nitride film 58 and the fourth silicon nitride film 61 serve as the supporting layers for supporting the three fins of the first, second and third conductive layers of the fin-structured storage electrode in order to prevent the three fins of the first, second and third conductive layers from being curved or bent to contact with each other. However, the supporting layers of the second, third and fourth silicon nitride films 55, 58 and 61 make it difficult to reduce the thickness of a capacitive insulation film covering the fin-structured storage electrode. The reduction in the thickness of the capacitive insulation film of the fin-structured storage electrode is essential to increase the capacitance of the fin-structured storage electrode, for which reason the difficulty in reduction in the thickness of the capacitive insulation film of the fin-structured storage electrode makes it difficult to increase the capacitance of the fin-structured storage electrode.
In order to settle the above problem, it is required to remove the second, third and fourth silicon nitride films 55, 58 and 61 by a wet etching or an isotropic etching. Since the etching rate of the silicon nitride film is low, a relatively long time is necessary for removal of the second, third and fourth silicon nitride films 55, 58 and 61 by the wet etching or the isotropic etching. In the light of mass productions, it is not suitable to increase the number of fins of the fin-structured storage electrode.
In order to increase the surface area of the fin-structured storage electrode, it is effective to increase a lateral length by which each of the fins extends laterally and outwardly from a column portion of the third polysilicon film. Namely, the lateral length of the fins is long as compared to a diameter of the contact hole 62. In order to increase the lateral length of the fins of the fin-structured storage electrode, it is needed to provide the silicon nitride films which support the conductive films serving as the fins and also prevent the fins from being curved or bent.
If, however, the diameter of the contact hole 62 is increased whilst the lateral length of the fins is reduced, then this suppress the fins from being bent even without support by the silicon nitride films. It is however impossible to decide the diameter of the contact hole without consideration of a pitch of low level interconnections, for example, word lines or bit lines. In other word, the diameter of the contact hole is decided by the pitch of the low level interconnections, for example, word lines or bit lines. In the light of increase in the density of the integration of the memory cell arrays or a possible reduction of the occupied area of the memory cell, it is required to set as narrow as possible the pitch of the low level interconnections, for example, word lines or bit lines. In order to obtain a maximum density of the integration or a minimum occupied area of the memory cell, it is required to set the pitch of the low level interconnections, for example, word lines or bit lines at the minimum scale. The diameter of the contact hole is required to be not larger than the pitch of the low level interconnections, for example, word lines or bit lines in order to prevent any short circuit. If, however, the diameter of the contact hole is larger than the pitch of the low level interconnections, for example, word lines or bit lines, then the contact hole is made into contact with the word lines or bit lines.
On the other hand, if the diameter of the contact hole is so small as compared to the lateral length of the fins, then the fin-structured storage electrode is likely to fall down or peel off.
In the above circumstances, it had been required to develop a novel fin-structured storage capacitor of the stacked memory cell capacitor free from the problems as described above and provide a novel method of fabricating the same.